Conventional scales for bathrooms or kitchen scales or the like are generally designed as spring balances. The balance platform moves upon being loaded a lever bridge system against the force of a proportionally deformed bias spring, said deformation reckoned from a zero postion being displayed. For the display for example a toothed rack is provided translatorily moved in response to the spring deformation, the rack engaging a pinion. The pinion carries a dial disc the relative position thereof with respect to a stationary mark being visible. Instead of a bias spring, the reactive force may be provided by a counterweight deflected in response to the load.
The dial scale is provided with a graduation indicating intervals of, for example, 0.5 Kg; intermediate values must be interpolated by the user which interpolation necessarily is inaccurate. For this reason it has been tried to provide the interpolation already within the scale and to display only the digital value most proximate to the weighing result, steps of, say, 0.5 Kg being provided.
As an example DT-OS 23 30 416 discloses a scale wherein the scale disc carries a code with respectively transparent and opaque areas respectively permitting and blocking the irradiation of photo transistors. By means of the photo transistors a light cipher display is controlled, the code being, of course, correspondingly designed. In this known scale, however, there is no definite display if the disc will come to a standstill just between two codes, so that depending upon the chosen dimensions it remains incidental whether the code is positioned sufficiently accurately between light source and photo transistor.
The scale disclosed in DT-OS 24 29 059 operates with motor servo regulation and counting of the revolutions of the motor shaft, the count being electronically displayed in digital fashion. This solution is unacceptable for bath scales and the like for reasons of the expenses.
A transducer to transform the mechanical length signal into an electrical digitally displayed signal is provided in the scale disclosed in FR-OS 73 24 097, however, this design being unsuited for household scales due to the inordinately high cost.
The scale disclosed in DT-OS 23 33 195 comprises a code plate instead of the scale disc, the code being formed by electrically conducting and insulating, respectively, surface portions. Upon standstill of the disc on termination of a weighing operation the cose is scanned by means of a set of electrically conductive collectors and thereafter electro-optically transducer by means of switched lamps and photo-transistors and finally displayed on a seven segment display. The code disc is provided with an indexing system, such that the disc may assume only certain preferred positions wherein the code is unambiguously scanned. This solution, however, suffers the unpermissable drawback of a high reaction to the weighing itself; if on the other hand one would design the indexing system with a force sufficiently small that the reaction could be neglected, this force would be insufficient to turn the disc into the preferred position.
Finally DT-OS 23 49 764 discloses a scale having no ambiguity of the display and having no reaction unto the weighing system. Again a code plate or drum provided with an optical code is scanned and the position of the code carrier relative to its rest position is digitally displayed. The code carrier is illuminated by means of one single light source and each segment of a multidigit sgement display is formed by the end of the fibre optical light guide. There is no indexing of the code carrier, the latter thus proportionally following the deflection of the weighing system. In order to exclude intermediate positions for the optical scanning the code carrier additionally is provided with an auxiliary code corresponding to a desired index. The auxiliary code, too, is optically scanned, and feeds, unless the position of the code carrier permits an immediate readout, a servo signal for a servo mechanism by means of which the light inlets of the light guides are moved relative to the scale housing and to the code carrier into the most proximate index position, the movement of the light guide inlets being analog to the indexing error or being performed in one predetermined step. Such a servo motor, of course, involves additional complexity and expenses and it must be assumed that the design is liable to failures over long term operation. The servo mechanism requires in each case additional energy source and the flexible fibre optical light guides are difficult to assemble.